The present invention relates to filtration systems which employ ion exchange resins as the filter media. In particular the invention relates to an improved process whereby undissolved solids which have been filtered from the water being treated may be removed from the upper portion of the ion exchange resin bed while the resins remain in an in line service vessel and without disturbing or reorientating the resins in the lower portion of the bed.
It is, of course, well known in the water filtration art that water demineralizers which employ deep beds of anion and cation exchange resins will remove from the water being purified therein both dissolved and undissolved impurities. Depending on the quantity and character of the water being treated, the bed of ion exchange resins may first be chemically exhausted or it may first be contaminated with undissolved impurities to the extent where excessive pressure drops are encountered. When the resins have been chemically exhausted, they must be chemically regenerated in order to restore their capacity to remove dissolved impurities. On the other hand, contamination of the resin bed with undissolved solids requires physical cleansing of the resins in order to return pressure drops to acceptable levels.
In some instances, and with particular reference to condensate polishing units in power generating systems, such deminerilizing units have their usefulness terminated by reason of the accumulation of undissolved impurities rather than by the attainment of a condition of ineffective ion exchange. Under such circumstances prior art processes generally utilize a backwashing technique wherein a liquid is passed through the bed of ion exchange resins in a direction reverse to that of the filtrate. In addition, some prior art processes, such as that disclosed in U.S. Pat. No. 3,455,819, introduce air or other inert gases into the bed to facilitate the cleaning or scrubbing of the resins.
Such prior art processes, however, suffer from several disadvantages. First, since the backwashing fluids are introduced at the bottom of the bed, the entire bed is disturbed and reorintated during backwash. This reorientation of the ion exchange resins can adversely effect chemical removal effectiveness, thereby lowering effluent quality. In addition, since the entire bed is being scrubbed, substantial amounts of backwash water must be utilized to conduct the process, thereby posing waste treatment problems, particularly in nuclear condensate systems.
Accordingly, the present invention provides a novel process for removing undissolved impurities from an ion exchange resin filtration unit. Significantly, it has been discovered that by scrubbing only the upper portion of the resin bed, where the concentration of such impurities is highest, acceptable pressure drops may be maintained and leakage of such impurities may be avoided without incurring the undesirable consequences of prior art processes outlined above. Thus, the greater portion of the resin bed remains undisturbed throughout the process and lesser quantities of backwashing liquids are required. In addition, since smaller quantities of resin are being scrubbed, the time required for undissolved solids removal is shortened thereby minimizing the time during which the unit is out of service.